Area Forecast Discussion
Issued by NWS Cheyenne, WY

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027
FXUS65 KCYS 211150
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
450 AM MST Tue Nov 21 2017

.UPDATE...
Issued at 447 AM MST Tue Nov 21 2017

Quick update to add the South Laramie Range into a winter weather
advisory. Radar trends show intermittent bands of snow training
over the area and web cams and WYDOT sensor indicate Interstate 80
is becoming snow covered near the summit. Snow bands will
diminish by mid morning with around 2 inches of new snow expected.
Snow covered roads will be slick and hazardous especially during
the early morning hours.

&&

.SHORT TERM...(Today and tonight)
Issued at 243 AM MST Tue Nov 21 2017

Upper level shortwave over southern MT will drop south across the
area through this morning. Associated 170kt NW`ly upper jet will
continue to provide some instability for banded snowfall along the
mountains (especially the Snowy`s) through this morning so kept
the Winter Weather Advisory in place. HRRR and other short range
guidance indicates a band of snow developing and tracking over the
South Laramie Range early this morning and this may be happening
based on trends from the Riverton radar. Surface and road temps
are fairly warm and any snowfall should be brief so not expecting
any major travel issues at this time, however trends will need to
be monitored in case there is a training band or the snow is heavy
enough to accumulate in spite of marginal temps.

Elsewhere, a surface cold front is currently moving south through
the plains with a band of light snowfall just behind it over the
extreme northern panhandle. Not expecting much in the way of
accumulation as temps are still above freezing and the ground is
warm. Clouds will increase over the remainder of the plains behind
the front this morning and slowly thin this afternoon. It will be
much cooler and less windy across many areas today.

Upper ridging will build over the intermountain west tonight with
increasingly subsident and anticyclonic flow increasing over SE
Wyoming late tonight. Surface lee troughing will redevelop with H7
flow increasing to 50kt over the S Laramie Range. Low level CAG-
CPR gradients increase to 58-64 meters by 12z Wed. With the mid
and low level flow being more WNW`ly think this is a set up that
favors Bordeaux and the S Laramie Range more than Arlington so
issued a High Wind Watch for these areas. Forecast soundings
indicate mtn wave potential with a decent inversion setting up
around ridge top level along with reverse shear w/height. While a
less sustained and more gusty event than this past one appears
likely, still think there`s the possibility for a few high wind
gusts late tonight.

.LONG TERM...(Wednesday through Monday)
Issued at 243 AM MST Tue Nov 21 2017

Winds are expected to be quite strong early on Wednesday AM across
the primary wind corridors of southeast Wyoming. Otherwise, do not
have any major weather concerns through mid-week as high-amplitude
upper level ridging develops over the western US. Wed & Thu should
be quite warm with the models showing 700 mb temperatures of +4 to
+6 deg C. Large scale ridging will quickly break down later in the
week. The GFS/ECMWF differ considerably in the timing, but both do
agree in a return to strong west-northwest flow aloft over the CWA
by Friday AM as a vigorous mid-level disturbance tracks rapidly to
the east across the northern high plains. As such, breezy to windy
conditions will return to much of the area by Thursday or Friday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1031 PM MST Mon Nov 20 2017

Strong wind speeds will persist across the KCYS and KRWL taf sites
through early tonight, before a frontal boundary starts to
approach the area towards daybreak. Before this front approaches
the Laramie Range, we will see the potential for moist upslope
flow which will may bring some snowfall to our higher elevations.
Not expecting much in the way of accumulation at KCYS and KLAR due
to these sites experiencing more downslope flow which will allow
for more drying. Although, cannot rule out some MVFR ceilings
towards daybreak. Meanwhile, the Nebraska TAF sites may also
experience some MVFR ceilings after the front passes through which
should begin to clear out around midday Tuesday. KCDR may also
experience some brief snowfall after the front passes through
around 12z.


&&

.FIRE WEATHER...
Issued at 243 AM MST Tue Nov 21 2017

Temperatures will be cooler and winds will be less today across
much of the area. Dry, warmer, and breezier weather return for mid
week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 11 AM MST this morning for WYZ114.

     Winter Weather Advisory until 9 AM MST this morning for WYZ116.

     High Wind Watch from late tonight through Wednesday morning for
     WYZ106-116-117.

NE...None.
&&

$$

UPDATE...DEL
SHORT TERM...DEL
LONG TERM...CLH
AVIATION...REC
FIRE WEATHER...DEL



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